Question

Why Ph scale is only up to 14

Answer 1

because till now there is no substance discovered whose pH is more than 14

Answer 2

In aqueous solution the autoionization of water limits this value according to 14=pOH+pH. For example the solubility of sodium hydroxide in water is about 111g/100mL or about 27M. pOH=-1.4 so pH=15.4.